When excessive beam current flows through projection TVs consisting of three CRTs for projecting red, blue, and green video images and a lens system, damage such as CRT burnout due to beam current, deterioration of luminous characteristics, and deterioration of lenses and lens coating due to heat generation may occur. (Hereafter referred to as deterioration of the CRT and lens).
To prevent this deterioration, conventional 3-CRT projection TVs detect the beam current of each CRT and measure the duration when the detected value exceeds a certain value. The duration is measured based on a circuit time constant using a resistance and capacitor. Then, in response to measurement results, a control circuit is employed to reduce the level of beam current. However, this type of control circuit suffers from too large a control error.
Conventional 3-CRT projection TVs which can project still pictures employ, for example, a circuit configured with a field discriminating circuit, an A/D converter, a D/A converter, and a field memory, which is same as that used for 1-CRT direct view TVs, as is disclosed in Japanese Laid-open Patent No. H5-344371 for preventing deterioration of the CRT and lens. Intervals with a smaller difference in fields are counted and a control voltage is output in response to the results.
This circuit also detects the beam current of each CRT and measures the duration when the detected value exceeds a certain value. The duration is measured based on a circuit time constant using a resistance and capacitor. The level of beam current is then reduced in response to the measurement results.
This method reduces the video contrast at a fixed rate due to the use of the circuit time constant. Therefore, in some cases, the time taken to detect the beam current for preventing deterioration of the CRT and lens before the beam current is reduced to a safe level for preventing CRT phosphor burnout may not be appropriate, resulting in the deterioration of the CRT and lens in some video patterns.
In addition, as 2-picture wide screen TVs and teletext broadcasting are becoming more popular, usage where the beam current is concentrated on either the left or right side of CRTs is increasing. A conventional circuit configuration is incapable of protecting CRTs under these usage conditions.
Furthermore, the circuit disclosed in the Japanese Laid-open Patent No. H5-344371 is expensive.